Keira Knightley plays a floundering young adult who becomes close friends with a teenager (Chloe Grace Moretz).

For her first outing as director without also being screenwriter, Lynn Shelton has chosen something with a plot that sounds a lot like one she'd imagine. Laggies, scripted by Andrea Seigel, is about a young woman, finding it impossible to move to the next phase of her life, who gets a jump-start by doing something highly unconventional and arguably stupid. The new film also moves away from the improv-heavy strategy of Shelton's early work (if there's any ad-libbed dialogue here, it doesn't show), a choice that suits the tightly plotted film; that, and a star-heavy cast featuring Keira Knightley, Chloe Grace Moretz, and Sam Rockwell, make Laggies the most movie-like movie Shelton has made, one that could easily see more box-office attention than all of her previous films combined.

Knightley plays Megan, a woman who attends her 10-year high school reunion and realizes very little has changed. She's still in her teenage clique, living with high school boyfriend Anthony (Mark Webber), and doing dumb jobs for her dad (Jeff Garlin) instead of using the degrees she earned. Her friends are getting married, pregnant and careerist; she's sneaking off to watch her parents' TV while claiming to see a career counselor.

When a perfect storm of anxiety-inducing events -- not least of which is a surprise proposal from Anthony, who is self-helping his way into a less procrastinatory life -- sends Megan wandering semi-aimlessly into the night, she crosses paths with a quartet of teens. Allowing 16-year-old Annika (Moretz) to convince her to buy them beer, she winds up spending half the night getting drunk with them.

Soon she decides she needs a vacation from her life. Telling Anthony she's off to a weeklong seminar he has recommended, she instead winds up crashing with Annika and spending more time with her friends -- something Annika's single dad Craig (Rockwell) finds difficult to understand.

It takes some funny eyes-askance interrogation from Rockwell to remind us just how questionable it is for a 28-year-old to start going to high school keggers and having slumber parties with a teenager. Knightley's performance, convincingly desperate for anything that might point in a promising direction, makes sense of it while it's happening. Seigel's script earns its share of laughs while drawing not-too-blunt parallels between the adolescent dramas Megan witnesses and the difficulties she herself is having. Some kind of romantic collision between Megan and Craig is inevitable from their first scenes together, and the film integrates that unwise attraction into teen-movie shenanigans nicely.

From production values to performances to the script, everything here is more polished than we've come to expect from Shelton's lovably loose films. Yet the finished product still feels of a piece with that body of work, emotionally optimistic and comfortable with characters who are rarely at ease.